From experience, Showsec are quite anti non-showsec crews having access into their systems. On the busier shows they do a pre-show briefing that covers callsigns in use and important features, and you won't be invited to those. Why would you want to talk to the security people, when the usual process would be to give one of your venue radios to their operator. I can understand you wanting to listen to them, but speaking is not appreciated. I did exactly this at a regular job of mine - programmed the venue's two channels backstage and foh into the radios we use for our crew, and it was made quite clear by 'short-circuiting' the usual process was not welcome. They didn't even like being listened to!